1. Field of the Invention
This description refers to a lifting system of pre-stressing cables inside of ducts in concrete structures. In particular, it relates to a lifting system for pre-stressing cables in concrete structures. More particularly for pre-stressed (pre-stressed) concrete structures of high altitude.
2. General Background
The pre-stressing of concrete structures requires the introduction of pre-stressing cables within ducts that are arranged inside the concrete segments mounted to form the structure. Once arranged the cables inside the ducts, cables are tensioned to exert a compressive force that holds together the concrete segments.
Several pre-stressed concrete structures are known in the art. For example, tanks (U.S. Pat. No. 4,045,929), cooling towers for nuclear reactors (U.S. Pat. No. 4,092,811); Silos (U.S. Pat. No. 1,834,842) chimneys (U.S. Pat. No. 5,038,540) and other elongated structures such as concrete towers or masts. In the case of concrete towers, they can be sectioned towers (EP-0 960 986A2, U.S. Pat. No. 7,114,295) or may be segmented towers (US-2005/0129504 and US-2006/0156681A1), or U.S. Pat. No. 7,739,843 and U.S. Pat. No. 8,555,600 by the same inventors hereof. Pre-stressing concrete structures over 30 meters high, have several drawbacks. In the prior art pre-stressing (post-tensioning) techniques, the pre-stressing cables are inserted into the ducts from the bottom of the structure through the use of motorized roller which force the cables to entry into the ducts and arise. This technique is effective particularly in mid-rise structures, 5-20 meters, however, in the case of structures for nuclear reactors or towers for wind power generators, which reach above 50 m heights, such method has several drawbacks. To reach the required height the motorized rollers require high power motors, due to the long distance the pre-stressing cables must run inside the ducts, it is common for them to jam. The problem is increased if the structure has a reduction of diameter or waist as in the case of nuclear reactors. Another way to place the pre-stressing cables is to upload a cable roll to the top of the structure using a crane, and insert the cable within the pre-stressing ducts. This embodiment involves a number of drawbacks. The handling of the roll is complicated and risky, because the main weight of the cable lies on the roll, but not in the end that is introduced in the duct, so that at all times there is a latent risk that the roll falls under its own weight and drag the cable, tools and operators in the fall. Moreover raising the cable roll carries a cost for the use of the crane that lifts the pre-stress cable roll.
Moreover, the layout of personnel or operators performing the post-tensioning is also a challenge. To perform the post-tensioning, operators should be placed on top of the tower, where the huge concrete blocks are stacked without a firm bond, at a height where there are high speed wind currents. Moreover, if the introduction of the wires is performed from the top of the tower to the bottom, the volume of equipment and pre-stressing cables become more insecure the work of the operators performing the pre-stressing.
A considered solution to this problem consists in placing strands inside the concrete column, adjacent to the interior wall, but not within ducts, see U.S. Patent-2005/0129504, FIG. 15 MECAL APPLIED. Such a solution has various drawbacks, for example, such configuration cannot be considered since the strands have a different nature and function with respect to the pre-stressing cables which are arranged embedded in the concrete structure, within ducts, moreover, the load to be applied to said strands is very low in relation to the loads that can be applied by pre-stressing. The tension collected in the pre-stressing cables ensure that the concrete tower works like a monolithic structure, in other words, as made in one piece.
It is therefore necessary an efficient and safe solution to the problem of lifting cables within ducts of pre-stressed concrete structures, particularly greater than 30 m height, more particularly, structures of between 80 and 150 m.